The present disclosure relates to a method and apparatus for improving performance of a solenoid assembly and, particularly, to an actuator assembly having an improved linear solenoid assembly for use in a motor vehicle.
In the newer known art, a linear actuator assembly includes primary and secondary pole pieces which cooperate to define an axially extending chamber in which is disposed a moveable armature. The armature includes a cylindrical member which moves, upon energization of the actuator, in the direction of the primary pole piece. The primary pole piece includes a substantially cylindrical center pole member with inner and outer walls defining a closed and an open end. The inner wall is substantially cylindrical and facilitates axial movement of the similarly configured armature, relative to the pole. As the armature moves in the direction of the closed end, a fixed, radial air gap is defined between the outer cylindrical wall of the armature and the inner cylindrical wall of the cylindrical center pole. Such a fixed air gap provides substantial controllability to the operation of the actuator.
It will be recognized that a solenoid assembly can be used in various actuator assemblies for actuation of a certain component and not limited to motor vehicles or internal combustion engines. One use for an actuator assembly having a linear solenoid involves cam phasing in an internal combustion engine of a motor vehicle, for example. Cam phasers are well known in the automotive art as elements of systems for reducing combustion formation of nitrogen oxides (NOX), reducing emission of unburned hydrocarbons, improving fuel economy, and improving engine torque at various speeds. As is known, under some operating conditions it is desirable to delay or advance the closing and opening of either the intake valves or the exhaust valves or both, relative to the valving in a similar engine having a fixed relationship between the crankshaft and the camshaft.
Typically, cam phasers employ a first element driven in fixed relationship to the crankshaft and a second element adjacent to the first element and mounted to the end of the camshaft in either the engine head or block. In modern automotive engines, the camshafts are typically disposed in the engine head for direct actuation of the valve tappets. Cam phasers are commonly disposed at the crankshaft and camshaft ends opposite the engine flywheel, at the xe2x80x9cfrontxe2x80x9d end of the engine. The first and second phaser elements are connected to cause the crankshaft to rotate the camshaft.
To provide a linear function to the operation of the actuator, the magnetic force acting on the armature is a function of input-amp turn of the coil, and is independent of the armature (i.e., plunger) position. However, current cam phase actuator designs provide a linear function only in a middle portion of plunger travel (approximately 2.0 mm travel distance) with a total travel of 3.0 mm and a maximum force of 14 N at 1400 amp-turns. In other words the force profile is not linear at beginning and ending travel portions of the plunger.
Currently, to approach a linear function in the operation of the actuator, the outer cylindrical wall of the cylindrical center pole is tapered outwardly, in the direction of the closed end thereof, such that as the armature moves in the direction of the closed end of the center pole, generally the translating direction of the solenoid operated rod member, the mass of the pole piece through which the magnetic flux is forced to pass increases, so as to control the rate of magnetic saturation necessary to provide the desired linear displacement versus current characteristic.
This current configuration results in a peak force intermediate of the ends of armature travel, which diminishes as the armature continues to move towards its maximum axial travel. Such a reduction in magnetic force as the armature, and associated rod member, approaches a fully opened position requires an increase in current to avoid a reduction in performance due to a loss of linear performance of the actuator.
A method and apparatus for a solenoid assembly for use with an internal combustion engine that addresses the reduction in magnetic force as the armature moves closer to the primary pole piece or stop. Force reduction is minimized and stroke length is increased by providing a novel, primary pole piece and armature configuration. The primary pole piece includes an inner tapered wall and an outer tapered wall with a flat section intermediate therebetween. The primary pole piece includes a L-shaped body with a substantially cylindrical center pole member for allowing translation of an actuating rod in operable communication with the armature. The inner wall, flat section, and outer wall define a frustoconical cavity configured to receive, for axial travel therein, the associated configured armature. The armature is configured having a conical portion on a periphery of the bottom surface of the armature for magnetic engagement with the frustoconical cavity formed in the primary pole piece. As the armature moves in the direction of the closed end of the L-shaped pole piece the mass of the pole piece through which magnetic flux may pass is increased thereby providing a linear function to the operation of the actuator. The inner tapered wall of the center pole member defines a semiconical end. The semi-conical end cooperates with a similarly tapered end on the armature periphery to establish a secondary air gap which is operable to increase the opening force on the armature across its range of motion as the force decreases at the primary air gap and, more importantly, as the armature nears its fully displaced location near the closed end of the axially extending chamber of the center pole member. As the armature moves within the axial chamber, leakage flux is directed from the wall defining the cylindrical shape of the armature to the inner tapered wall of the center pole member providing an additional force component in the axial direction. As the tapered end of the armature approaches the closed end of the axial chamber, leakage flux is directed across the secondary gap defined by the associated tapered surfaces of the inner tapered wall and the armature to rapidly compensate for the decreased force component in the axial direction from the primary gap and thereby compensate for the force reduction experienced in prior linear actuators.